The growth in the installed base of personal computers and the emergence of new data communications services testify to the potential for growth in the volume of on-line communications. Although the current situation is satisfactory for large-scale users who are able to connect their internal networks to external services via high-speed transmission networks or leased lines, the cost of such solutions is too high for small businesses or individuals.
It seems certain that this situation will change, and that the necessity of distributing services involving a high transmission rate right to private residences will be at the origin of a widespread broadband network. However, there is little chance of such a network being deployed globally for several years, or even tens of years.
While awaiting the availability of such a high-speed network, individuals, schools and small businesses have to resort to transmitting data on the telephone network. To do that, they have the choice between two known technologies. The most widespread solution consists in using modems operating at a rate of 14.4 kb/s or 28.8 kb/s. A more expensive solution consists in using the ISDN network, which has two data channels at 64 kb/s and a signaling channel at 16 kb/s. However, this second solution is not available everywhere, and requires substantial investment for the users in the form of higher subscriber tariffs and suitable communications hardware.
A modem is known from EP-A-0 669 740 from AT&T Corp. which is intended to be used with analog subscriber lines at the two ends of the transmission path. This modem comprises means for separately equalizing each loop of an analog telephone network connection, by using a plurality of transmission filters and a plurality of reception filters. The filters are used in such a way that, in the direction of transmission from the modem to the exchange, the voltage samples sent by the coding/decoding stations are equivalent to the quantization levels of the exchange.
The need to use prescribed levels for the quantization law makes it impossible to choose the levels which have the best resistance to noise in the presence of an echo signal.
Furthermore, the above document does not describe a device for communication between a digital adapter and an analog adapter, in which the digital adapter would be linked to an exchange via a digital interface.